Method for correcting the measuring errors of a hot-film air-mass meter

ABSTRACT

A method for correcting a measuring error of a hot-film air-mass meter occurring as the result of backflow, in particular for acquiring the air mass of the combustion air of an internal-combustion engine. It is proposed for an errorfree acquisition that the air volume be measured with the hot-film air-mass meter as a first value (23) and further as a second value 24 with a second method for determining air volume (α/n method), which works independently of the hot-film air-mass meter; that the two values (23, 24) be drawn upon alternatively as a valid variable dependent upon the operating ranges determining the measuring reliability; and that in at least one operating range which is free of backflow for a correction signal (K H ) to be extracted from a comparison of the first (23) and the second value (24) and used to correct the second value (24) in operating ranges showing backflow.

FIELD OF THE INVENTION

The present invention relates to a method for correcting a measuringerror of a hot-film air-mass meter occurring as the result of backflow.

BACKGROUND INFORMATION

Hot-film air-mass meters can be used to detect the air mass drawn in byinternal-combustion engines. These meters have a heated element, whichis situated in the air flow to be measured and is cooled in this manner.In particular, it is possible to use the heated element as part of anelectric bridge circuit and to keep it at a constant overtemperaturerelative to the intake air temperature by means of a current flowingthrough this element. With this principle, the required heating currentis a measure for the air mass drawn in by the engine. The pulsations ofthe intake air, which possibly occur in certain operating ranges of aninternal-combustion engine, can cause the measuring result to becorrupted. This is particularly the case when a so-called backflowoccurs, since the hot-film air-mass meter cannot distinguish thedirection of flow.

It is generally known to provide a hot-film air-mass meter with anevaluation circuit, so that a backflow can be recognized usingprogramming technology. A high computing power is required for this. Thebackflow is recognized from the evaluation of the signal waveshape.

Furthermore, when backflows occur, it is known to adjust the outputsignal from a hot-film air-mass meter with a correction value. Incertain operating ranges, however, only a very inaccurate result is ableto be attained.

SUMMARY OF THE INVENTION

In contrast, the method according to the present invention has theadvantage that even when backflows occur, there is a high degree ofaccuracy in detecting the air mass and thus the load value of theinternal-combustion engine. It is not necessary to evaluate the signalwaveshape, which is relatively costly and requires an appropriatecomputing capacity. Rather, the air mass measured with the hot-filmair-mass meter is defined as a first value and compared to a secondvalue, which is determined using another independently working methodfor determining air volume. Dependent upon the operating range existingat the time, either one or the other value is drawn upon as a validvariable that determines the air mass. Preferably, the air mass isdetermined per stroke of the internal-combustion engine, which, in thecase of the internal-combustion engine provided with an injection systemand stoichiometric combustion, is proportional to the injection time ofthe injection valves bringing in the fuel. In order to always be able toadapt individually to the conditions existing at the time, in accordancewith the present invention, a correction signal is extracted in at leastone backflowfree operating range from a comparison of the first and thesecond value and used to correct the second value in operating rangesthat have backflow. Thus, the present invention involves applying thevalue acquired by the hot-film air-mass meter in certain operatingranges and, in other operating ranges in which the value acquired by thehot-film air-mass meter is inaccurate, working with another value thathad been determined on the basis of a different method for determiningair volume, whereby errors occurring with the mentioned method fordetermining air volume are corrected using an adaptation method. Thecorrection signal which enables the adaptation is extracted thereby bycomparing the first and the second value in an operating range which isfree of backflow. The invention therefore makes use of the realizationthat no backflow occurs in certain operating ranges, so that thehot-film air-mass meter supplies correct data. These data constitute thebasis, namely a calibration value, for the result determined accordingto the second method for determining air volume. In this respect, inranges in which the hot-film air-mass meter supplies results which areinaccurate due to backflow, a very high accuracy is achieved by means ofthe adaptation according to the present invention with a method fordetermining air volume which functions on the basis of other principles.

A further development of the present invention provides for thecorrection signal to be an altitude-correction signal. Therefore, themeasuring result of the second method for determining air volume isdependent on altitude, so that a correction must be made in order toavoid measuring errors. By using the altitude correction, one obtainsthe air mass from the air volume that is determined.

Preferably, in the case of the second method for determining air volume,the throttle-valve angle and the rotational speed of theinternal-combustion engine are drawn upon and subjected to anengine-characteristics-map and/or algorithm processing to determine thesecond value.

Preferably, the measured value of the hot-film air-mass meter is appliedas a valid variable in no-load operation, when the throttle-valve anglesare small, and when the rotational speed is high. The operating rangesassumed in this case guarantee an error-free measuring result for thehot-film air-mass meter. With regard to the mentioned high rotationalspeed, it can be said that for rotational frequencies higher thanapproximately 3000 per minute, backflow no longer occurs. This limitingspeed is dependent on the particular geometry of the suction pipe. Sincealso the pressure of the suction pipe at rotational speeds of <3000 perminute, already at a relatively small throttle-valve angle, no longerincreases when the throttle valve is further opened and, moreover, thethus characterized limiting angle is still a function of the rotationalspeed, this relatively complicated correlation shall be described by acharacteristic curve (limiting characteristic curve). This is determinedin that a limiting angle exists, which preferably corresponds to 95% ofthe full load, that is, of the maximum suction-pipe pressure. If thethrottle-valve angle existing at the moment is smaller than the limitingvalue which can be drawn from the engine characteristics map for therotational speed existing at the moment or can be calculated using thealgorithm, then an operating range exists for the internal-combustionengine in which no backflow can occur. Consequently, the measured valueof the hot-film air-mass meter is applied as a valid variable in theseoperating ranges. However, if the throttle-valve angle is larger and theinternal-combustion engine is in a rotational-speed range which liesbelow the mentioned limiting speed, then the engine is in a full-loaduseful range, in which backflow is possible. In this range then,according to the present invention, it is not the measured value of thehot-film air-mass meter which is applied as a valid variable, but ratherthe measured value of the second method for determining air volume inview of the depicted adaptation.

In summarizing, it can be said, therefore, that the measured value ofthe hot-film air-mass meter is retrieved as the valid variable in thecase of working points which lie below the limiting characteristic curveof the throttle-valve angle/rotational speed diagram, whereby thelimiting characteristic curve lies preferably in the upper load range,in particular in the range between 60 and 95% of the full load.Furthermore, the measured value of the hot-film air-mass meter is usedas a valid variable for rotational speeds which lie above a limitingspeed of preferably 3000 revolutions per minute.

Since slight changes in very small throttle-valve angles bring aboutvery large changes in the volumetric flow in the suction pipe, thisoperating range is not suited for generating the correction signal andtherefore is not suited for the adaptation. As a result, the adaptationis preferably not carried out in the mentioned range.

If the no-load setting of the internal-combustion engine is achieved bymeans of a by-pass controller, the second value is corrected by theamount of the no-load partial-air mass, which was not determined by thethrottle-valve angle. This is the only way an error-free result can beattained.

To generate the correction signal, the difference between the valuesdetermined by the various methods for determining air volume istransmitted to an integrator, whose output value is fed to a multiplieras a first input quantity, whereby the second input quantity of themultiplier is the second value determined according to the second methodfor determining air volume. So long as there is a difference between thetwo values, the integrator is "integrated up or down" accordingly.

A further development of the present invention provides for a dynamictransition compensation variable to be produced from the correctedsecond value and superimposed on the valid variable. This transitioncompensation has the task of compensating for the time delay occurringwhen fuel is transported caused by dynamic quantities of additional orless fuel. The time delay comes about because the fuel volume injectedinto the suction pipe does not arrive directly in the correspondingcylinders of the internal-combustion engine, but is first "caught" onthe inner wall of the suction pipe. Only in the course of subsequentwork cycles of the internal-combustion engine is the correct fuel volumeregulated in the cylinder, corresponding to the existing working pointof the internal-combustion engine. The deficiency or excess occurringduring the transition performance is compensated for by the dynamictransition compensation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a circuit arrangement for the methodaccording to the present invention;

FIG. 2 shows a diagram which clarifies the temporal action of the airflow existing in the suction pipe of an internal-combustion engine.

FIG. 3 shows a diagram corresponding to FIG. 2 with an air flow whichpulsates heavily and exhibits backflow.

FIG. 4 shows a throttle-valve-angle/rotational speed diagram with a 95%full-load limiting characteristic curve.

FIG. 5 shows a schematic representation of a suction pipe with ano-load, by-pass.

FIG. 6 shows a schematic representation of the cylinder-head area of acylinder of the internal-combustion engine.

According to FIG. 1, the air mass m_(HFM) detected per unit time by thehot-film air-mass meter not depicted) is supplied to a multiplicationpoint 1, where the cycle duration TD of a suction period of aninternal-combustion engine is applied as a further input signal. Theinjection time TL_(HFM) determined by the hotfilm air-mass meter isavailable with the output quantity 2 of the multiplication point 1 andit corresponds to a specific fuel mass per stroke. Assuming astoichiometric combustion, the injection time TL_(HFM) is proportionalto a corresponding air mass per stroke. The injection time TL_(HFM) isfed to a subtraction point 3, which is furthermore supplied with ano-load injection time TL_(LL). This no-load injection time TL_(LL)corresponds to a specific air volume per stroke, which must be madeavailable in a by-pass situated parallel to the throttle valve for theno-load adjustment (compare FIG. 5).

The initial value 4 of the subtraction point 3 is fed to a pole 5 of acircuit element 6. The other pole 7 of the circuit element 6 isconnected to a summing point 8.

Furthermore, the initial variable 2 corresponding to the injection timeTL_(HFM) is connected to a pole 9 of a changeover switch 10, which canpreferably be actuated at the same time as the circuit element 6 bymeans of an operative connection 11. The actuation is achieved by acontrol circuit 12, which shall be clarified in greater detail in thefollowing.

A pole 14 of the changeover switch 10 which has a changeover contact 13is connected to a multiplication point 15, which receives a correctionfactor K_(Lambda) obtained from a Lambda feedback control as a furtherinput quantity. Thus, in the case of the internal-combustion engine,when Lambda is not equal to one, that means that a non-stoichiometriccombustion exists. This is then allowed for by the correction factorK_(Lambda).

The output 16 of the multiplication point 15 is connected to a summingpoint 17 to supply an input quantity. As a second summand, the summingpoint 17 receives an initial value 18 from a transition-compensationcircuit 19. The injection time TL is available at the output 20 of thesumming point 17.

The throttle-valve angle α and the rotational speed n (actual rotationalspeed) of the internal-combustion engine are fed to an enginecharacteristics map 21, which supplies an injection time TL_(DK) as anoutput quantity 22, which is dependent upon the throttle-valve angle αand the rotational speed n. The injection time TL_(DK) determined inthis manner is proportional to a corresponding air volume per stroke.Therefore, the air mass or air volume is acquired using two differentmethods; on the one hand with the already described hot-film air-massmeter, which measures air mass and, on the other hand, by means of thethrottle-valve-angle rotational-speed engine characteristics map 21,which makes it possible to determine the air volume. The air masscorresponding to the injection time TL_(HFM) represents a first value 23and the air volume corresponding to the injection time TL_(DK)represents a second value 24. While in the case of the hot-film air-massmeter, the density of the air is allowed for in principle, this is notthe case with the α/n method. Thus, as already explained, the actual airmass is acquired with the hot-film air-mass meter. However, only the airvolume is acquired with the α/n method, and this air volume must becorrected for altitude (corrected for density) to determine the airmass.

The injection time TL_(DK) is fed to a multiplication point 25. Aninitial value 26 of an integrator 27, which is connected by its input toan output 28 of the summing point 8, is supplied as a further factor tothe multiplication point 25. The output 29 of the multiplication point25 leads to a further input 30 of the summing point 8. Since the signalcoming from the multiplication point 25 is fed with a positive sign andthe signal coming from the circuit element 6 is fed with a negative signto the summing point 8, the difference between the two signals isavailable at the output 28.

Furthermore, the output 29 of the multiplication point 25 leads to asumming point 31, which receives the already mentioned injection timeTL_(LL) as a further input quantity. The output 32 of the summing point31 leads to a pole 33 of the changeover switch 10. In addition, theoutput 32 is connected to an input 34 of the transition-compensationcircuit 19.

FIG. 2 depicts the air flow in the suction pipe of theinternal-combustion engine as a function of time. One can clearlyrecognize that the air mass pulsates per unit time (m). This means thatthere is no continuous flow. The pulsation is a reaction to the workingcycles of the internal-combustion engine which do not followcontinuously, but rather in cycles. The cycle duration TD of one suctionperiod lies between every two points of ignition.

In certain operating ranges of the internal-combustion engine, thepulsation can become so great that backflow occurs. This means that theair-mass current reverses its direction in the suction pipe. Thebackflow is shown as the shaded area in FIG. 3. Since the hot-filmair-mass meter cannot detect the direction of flow, the air mass flowingback is also detected as positive, so that a measuring error occurs. Thehot-film air-mass meter measures the shaded areas in FIG. 3 as airmasses supplied to the internal-combustion engine; this is indicated bya dot-dash line in FIG. 3. The errors occurring in this respect as aresult of the hot-film method for detecting air volume are eliminated bythe method according to the present invention, which is clarified ingreater detail in the following.

The control circuit 12 actuating the circuit element 6 and thechangeover switch 10 through the operative connection 11 has a limitingcharacteristic curve in accordance with the diagram of FIG. 4. Thethrottle-valve angle α is plotted on the ordinate of the diagram and therotational speed n of the internal-combustion engine is plotted on theabscissa. The load of the internal-combustion engine is dependent uponthe throttle-valve angle α to such an extent that already at relativelysmall throttle-valve angles α for small rotational speeds, thesuction-pipe pressure no longer increases when the throttle valve isfurther opened up. Moreover, the throttle-valve angle α is still afunction of the rotational speed n. The characteristic curve of FIG. 4describes the rotational-speed dependency of a limiting angle, which isfixed so that its setting corresponds to 95% of the full load. Inaddition, the diagram of FIG. 4 shows a rotational-speed boundary linen_(Grenz) and a throttle-valve-angle boundary line α_(Grenz). Thepresent invention makes use of the fact that no backflow occurs in theshaded area in FIG. 4. This means that to acquire the injection time TLor the air mass per unit time, the measured value (first value 23)acquired by the hot-film air-mass meter can be used. The mentioned arealies below the 95% limiting characteristic curve and is defined by therotational-speed boundary line n_(Grenz) and the throttle-valve-angleboundary line α_(Grenz). Working points which lie above the 95% limitingcharacteristic curve (such as the working point a) require an air-massacquisition that is not carried out by the hot-film air-mass meter,because measuring errors occur. In this case, the just-mentioned secondmethod for determining air volume is employed. The acquisition of thethrottle-valve angle and the rotational speed, as well as the enginecharacteristics map 21, aid in carrying out this method. Therefore, aTL_(DK) injection-time acquisition is undertaken for the mentionedworking point.

The working point b sketched in FIG. 4 lies within the shaded area.Since no backflow occurs here and consequently the hot-film air-massmeter works in an error-free manner, the air mass can be acquired bymeans of the hot-film air-mass meter. For working points which show avery small throttle-valve angle (working point c in FIG. 4), it appliesthat very small changes in the throttle-valve angle α already lead torelatively large changes in the volumetric air flow. This presupposesangle-setting detectors of an especially high quality for the throttlevalve, which in addition must work in a manner that is free from playand is therefore very expensive. Since an inexpensive solution isaspired to with the method according to the present invention and,therefore, the throttle-valve setting is accomplished with a standardpotentiometer, this throttle-valve angular dimension is not used toadapt the injection time TL_(DK), as described in the following ingreater detail. On the other hand, however, the air mass is acquiredwith the hot-film air-mass meter within the range of smallthrottle-valve angles α existing here and, in particular, within theno-load range as well.

No backflow is possible in the case of rotational speeds which lie abovethe rotational-speed boundary line n_(Grenz). In this respect, theacquisition of air mass is achieved within this range by means of thehot-film air-mass meter.

The control circuit 12 actuates the circuit element 6 and the changeoverswitch 10, depending on the position of the working point existing atthe moment, in a way that enables the air mass to be acquired by thehot-film air-mass meter in the operating range which is free ofbackflow. When backflow occurs, thus at working points which lie abovethe 95% limiting characteristic curve, the other, second method fordetermining air volume is drawn upon for acquiring the air volume, orrather for acquiring the injection time TL_(DK) in proportion to theacquisition of the air volume. This second method works with thethrottle-valve angle α and the rotational speed n, as well as the enginecharacteristics map 21. The switch positions of the circuit element 6and of the changeover switch 10 depicted with a dotted line in FIG. 1correspond to an operation in which the hot-film air-mass meter isemployed. The initial value of the hot-film air-mass meter (m_(HFM)) ismultiplied at the multiplication point 1 by the cycle duration TD of onesuction period, and the thus formulated injection time TL_(HFM) is fedvia the changeover switch 10 to the multiplication point 15. In thiscase, a multiplication is carried out with the correction factorK_(Lambda), which is obtained from the Lambda feedback control, asalready mentioned. The value which is available at the output 16 of themultiplication point 15 is then transmitted via the summing point 17 tothe output 20. Accordingly, the injection time TL determined in thismanner is based on the measurement of the hot-film air-mass meter.

Since the circuit element 6 is in a closed state in the previouslydescribed operating range, the injection time TL_(HFM) is furtherconveyed via the subtraction point 3 to the summing point 8. Thethrottle-valve angle α existing in the specific working point of theinternal-combustion engine, as well as the corresponding rotationalspeed n, are likewise sent via the engine characteristics map 21 and themultiplication point 25 to the summing point 8. Consequently, the firstvalue 23 (TL_(HFM)) is compared to the second value 24 (TL_(DK)) at thesumming point 8. These two values are compared in order to generate acorrection signal K_(H) at the output of the integrator 27. Thiscorrection signal K_(H) allows for the influence of altitude to whichthe load (TL_(DK)) of the α/n engine characteristics map is subjected.If one did not make this altitude correction, then the second value 24would be inaccurate. The error amounts to about 10% per 1000 meters ofaltitude.

The altitude correction takes place according to an adaptive method ofthe present invention. This means that for operating ranges in which nobackflow occurs, the first value 23 is constantly compared to the secondvalue 24, and the correction signal K_(H) is determined from thiscomparison. If an operating range encumbered with backflow issubsequently started by the internal-combustion engine, then on the onehand the control circuit 12 switches over the circuit element 6 as wellas the changeover switch 10 in a way that enables the transition to bemade from the acquisition of the air mass using the hot-film air-massmeter to the α/n acquisition. Therefore, the circuit state exists asrepresented by the contacts depicted with a solid line in FIG. 1. Theinjection time TL_(DK) is supplied in this case to the multiplicationpoint 25 and adaptively corrected accordingly by means of the correctionsignal K_(H). The no-load correction takes place then still at thesumming point 31. The thus determined injection time is transmitted viathe changeover switch 10 to the multiplication point 15. The Lambdacorrection is made there and finally the injection time TL is availableat the output 20. The result of the adaptation is that in the case ofthe correction signal K_(H), one works with a value which was determinedshortly before switching over from hot-film air-mass-meter operation toα/n operation. Therefore, a system is obtained which adapts to theactual conditions and compensates for altitude errors.

An exception can be made in so far as the adaptation is prevented foroperating ranges with a very small throttle-valve angle α, since, asalready described, this would require a high-resolution potentiometer todetect the throttle-valve setting. In this embodiment, therefore, thecircuit element 6 is not rigidly coupled to the changeover switch 10,rather the circuit element 6 is switched separately, independently ofthe circuit state of the changeover switch 10.

FIG. 5 shows a section of the suction pipe 36 that contains the throttlevalve 41. The throttle valve 41 is bridged over by a by-pass 42 with aby-pass controller 43, so that the no-load adjustment can be made forthe internal-combustion engine in this manner. In this respect, thehot-film air-mass meter configured in the vicinity of the throttle valve41 cannot acquire the partial volume of air passing through the by-pass42. Therefore, according to FIG. 1, the configuration according to thepresent invention makes a corresponding correction (TL_(LL)).

It is further apparent from FIG. 1 that the injection time value(TL_(DK)) corrected by the altitude-dependent adaptation is supplied viathe summing point 31 to the transition-compensation circuit 19independently of whether the first method for determining air mass(hot-film air-mass meter) or the second method for determining airvolume (α/n method) is in use. The output value 18 from thetransition-compensation circuit 19 is always added to the value from theoutput 16 of the multiplication point 15 with the help of the summingpoint 17 to generate the injection time TL (output 20). However, anoutput value 18 occurs only in the case of dynamic transitions, that is,when an appropriate transition correction is required with respect tothe supplying of air volume because of fuel variation quantities"getting caught" with time delay at the wall of the suction pipe. FIG. 6illustrates the time delay for the fuel. The fuel 37 brought in from theinjection valve 36 depicted there into the suction pipe 35 remainspartially as a coating 38 on the inner wall of the suction pipe andenters only with a time delay through the intake valve 39 into thecylinder 40.

In the ranges in which the hot-film air-mass meter is drawn upon todetermine air mass, the method according to the present invention hasthe advantage of high accuracy, so that the load value can also bedefined very accurately. An altitude error does not occur in this case.When backflows occur, the second method for determining air volume isprovided, which is adaptively corrected for altitude (densitycorrection). Here, to measure the throttle-valve angle α, a simple,single-track potentiometer can be used. Its accuracy suffices becausethe injection time TL_(DK) is retrieved as a valid quantity only inthose operating states in which large volumes of air are relocated.Thus, the requirements for the resolution and the linearity of thepotentiometer that is used can be reduced.

Besides this, the described transition compensation is made with TL_(DK)values. They are provided considerably faster than the values from thehot-film air-mass meter, as this measuring instrument possesses acertain inertia. One thus obtains a transition compensation that has avery short reaction time.

What is claimed is:
 1. A method for determining the air mass flowinginto an engine, comprising:measuring a first value of the air mass withan air-mass meter; determining a second value of the air massindependently of the air-mass meter; comparing the first value to thesecond value during a backflow-free operating range of the engine todetermine a correction signal based thereon; and adjusting the secondvalue based on the correction signal during an operating range of theengine in which backflow occurs.
 2. The method according to claim 1,wherein the correction signal is selected from a group including analtitude-correction signal and a temperature-correction signal.
 3. Themethod according to claim 1, wherein the step of determining the secondvalue includes the step of supplying a throttle-valve angle and arotational speed of the engine to an engine-characteristics-map oralgorithm-processing unit.
 4. The method according to claim 1, whereinthe first value is treated as the actual air mass in no-load operationof the engine.
 5. The method according to claim 1, wherein the firstvalue is treated as the actual air mass when a throttle-valve angle issmall and when a rotational speed of the engine is high.
 6. The methodaccording to claim 1, wherein the first value is treated as the actualair mass when a throttle-valve angle is smaller than a limiting angle.7. The method according to claim 1, wherein the first value is treatedas the actual air mass when a rotational speed is higher thanapproximately 3000 revolutions per minute.
 8. The method according toclaim 1, wherein the second value is adjusted based on a no-load partialair mass if a by-pass controller is utilized.
 9. The method according toclaim 1, further comprising the steps of:integrating a differencebetween the first value and the second value; and multiplying the secondvalue by the integrated value.
 10. The method according to claim 1,further comprising the steps of:determining a dynamictransition-compensation value based on the adjusted second value; andadding the dynamic transition-compensation value to a value selectedfrom a group including the first value and the second value.